( i08 ) 



displcieement of 0,1 mm. is still noticeable, as will appear from the 

 discussion of the plates, currents of 10 -'^ Amp. can consequently be 

 detected. 



As far as is known to the writer, no other galvanometer is capable 

 of demonstrating with certainty such feeble currents. In practical 

 work the string galvanometer must consequently be placed on a line 

 with the most sensitive galvanometers of other construction and must 

 be distinguished from so-called oscillographs which only react on much 

 stronger currents. 



The force which deflects the string in a field of 20 000 C. G. S. 

 with a current of 10 -^- Amp. is x'ery small and works out for a 

 length of 12.5cm. at 2.5X10 ^^^ gi-ammes i.e. four times less than 

 one ten millionth part of a milligramme. 



By giving the string a greater tension its movements become quicker 

 but its deflections for equal currents less. It is easy to give the string 

 exactly such a tension that a current of given intensity causes a 

 predetermined deflection, as may appear from the photograms of the 

 two accompanying plates. These photograms were obtained in the 

 same way as the foniiorly described capillary-elect rometric curves ^). 



The 660-fold enlai-ged image of the middle part of the sti-ing is 

 projected on a slit, perpendicular to the image. Before the slit a 

 cylindrical lens is placed, the cixis of which is parallel to it ; behind 

 it a sensitive plate is moved in the direction of the image of the 

 string. While the movements of the string are thus registered, at the 

 same time a system of coordinates is pi'ojected on the sensitive plate 

 by the excellent method of Garten ^). Of these coordinates the hori- 

 zontal lines are obtained by mounting a glass millimetre-scale close 

 before the sensitive plate so that the sharp shadows of the scale- 

 divisions fall on the plate, while the vertical lines owe their origin 

 to a uniformly rotating spoked disc which intermittently intercepts 

 the light falling on the slit. The distance of the vertical as well as 

 of the horizontal lines has in our photograms been taken about one 

 millimetre, every fifth line being somewhat thicker. This latter pecu- 

 liarity can easily be introduced into the coordinate system by drawing 

 every fifth line in the glass millimetre-scale before the sensitive plate 

 slightl}" thicker and by also making every fifth spoke of the rotating 

 disc somewhat broader. 



1) See various essays in "Pflüger's Arch. f. d. gesammte Physiol." and in 

 "Onderzoekingen physiol. laborai. Leyden." 2nd series. 



2) Dr. Siegfried Garten. Ueber rhythmische elektrische Vorgiinge im querge- 

 streiften Skeletmuskel. Abhandl. der Königl. Sachs. Gesellsch. der Wissensch. zu 

 Leipzig. Mathem. phys. Glasse, Bd. 26, No. 5. S. 331. 1901. 



